A cheesy "hip" portmanteau invented by the media during the mid-2000s to describe a new form sexual expression that had come about due to increased pressure and stifling of sexual freedom imposed by the hypocritical older generation (which they did not face during their younger years).

Origin

Early 21st century: blend of sex and text.

https://en.oxforddictionaries.com/definition/sext

Used to increase legislation targeted mostly at imprisoning 18-25 year old males and create a moral panic among already overbearing parents, both of which are much easier to do when you can wrap it up in one catchy word that you think sounds like something the youngsters would say.